2. Solar Flare
Another threat would be a solar flare, which could instantaneously knockout earth's power grid.
The sun works on 11-year cycles and when it reaches its "solar max", it generates "Coronal Mass Ejections," which are basically large bubbles of gas producing electromagnetic discharges that can mess with our highly interconnected electric systems.
According to NASA, a large sun storm could reach the earth in minutes and cause power failures that would leave 130 million people in the dark in the U.S. alone, and cost trillions of dollars in damage and take years to fix. A worst-case scenario might mean that international communications, transportation and trade would be cut off, and food supply would be disorganized. The next major period of solar storms could occur between 2012 and 2014.